The invention relates to a method for producing directionally solidified and monocrystalline structures, in particular from super-alloys.
Metallic workpieces with monocrystalline structures are used as parts of machines which are exposed to high mechanical, thermal and/or chemical stresses during operation. For example, the blades of gas turbines, in particular also those of rotors for airplane engines, but also those for stationary gas turbines, are manufactured of monocrystals. The manufacture of monocrystalline workpieces of this kind is done e.g. by directional solidification from the melt. These are casting processes in which the liquid metallic alloy solidifies to a monocrystalline structure, i.e. to a monocrystalline workpiece.
A special casting process for the manufacture of such workpieces is for example known in which the liquid alloy, which is located in a ceramic mold, obtains a crystal orientation in a directed temperature field, for example of a Bridgeman furnace. In this method the dentritic crystals are aligned along the thermal flux and form either a columnar crystalline grain structure (i.e. grains which extend over the full length of the workpiece and here, in accordance with the general linguistic usage, are termed "directionally solidified"), or a single crystal structure, i.e. the whole workpiece consists of a single crystal. In this process one must avoid the transition to the globular or globulitic (polycrystalline) solidification because this non-directed growth necessarily forms transverse and longitudinal grain boundaries, which negate the good characteristics of the directionally solidified or single crystal component.
When mention is made of monocrystalline structure and monocrystalline structures in the present specification, this is meant to cover both monocrystals (single crystals) which have no grain boundaries and crystal structures which have grain boundaries which extend longitudinally but no grain boundaries which extend in the transversal direction. In the second named crystalline structures one also speaks of directionally solidified structures.
When talking generally of directionally solidified structures, this means both single crystals, which have no grain boundaries, or at most small-angle grain boundaries, as well as columnar crystal structures which do have grain boundaries extending in the longitudinal direction but no transverse grain boundaries.
So-called super-alloys on a nickel (Ni), cobalt (Co) or iron (Fe) basis are used amongst others as alloys, for example for the above-mentioned monocrystalline turbine blades. Especially super-alloys on a nickel basis have excellent mechanical and chemical high temperature properties.